Inter-locus antagonistic coevolution as an engine of speciation: Assessment with hemiclonal analysis

One of Ernst Mayr's legacies is the consensus that the allopatry model is the predominant mode of speciation in most sexually reproducing lineages. In this model, reproductive isolation develops as a pleiotropic byproduct of the genetic divergence that develops among physically isolated populations. Presently, there is no consensus concerning which, if any, evolutionary process is primarily responsible for driving the specific genetic divergence that leads to reproductive isolation. Here, we focus on the hypothesis that inter-locus antagonistic coevolution drives rapid genetic divergence among allopatric populations and thereby acts as an important "engine" of speciation. We assert that only data from studies of experimental evolution, rather than descriptive patterns of molecular evolution, can provide definitive evidence for this hypothesis. We describe and use an experimental approach, called hemiclonal analysis, that can be used in the Drosophila melanogaster laboratory model system to simultaneously screen nearly the entire genome for both standing genetic variation within a population and the net-selection gradient acting on the variation. Hemiclonal analysis has four stages: (i) creation of a laboratory "island population"; (ii) cytogenetic cloning of nearly genome-wide haplotypes to construct hemiclones; (iii) measurement of additive genetic variation among hemiclones; and (iv) measurement of the selection gradient acting on phenotypic variation among hemiclones. We apply hemiclonal analysis to test the hypothesis that there is ongoing antagonistic coevolution between the sexes in the D. melanogaster laboratory model system and then discuss the relevance of this analysis to natural systems.